ANALYSIS OF LONG-TERM BEHAVIOR OF ULTRAVIOLET-RADIATION MEASURED BY ROBERTSON-BERGER METERS AT 14 SITES IN THE UNITED-STATES

Surface ultraviolet (UV) radiation measurements from the Robertson-Ber l (RB) meter network and existing documentation of these data were exa mined to determine long-term variations of UV. RB meter data from 14 s ites in the United States were analyzed for trends over the period 197 4-1991. A more in-depth analysis of the RB meter data, including the u se of supporting geophysical data, was carried out for four of the loc ations. Results based on analysis of data from the 14 sites show a sig nificant negative trend of the order of -6% per decade overall, reason ably consistent with annual trends obtained by Scotto et al. [1988] us ing similar data for the period 1974-1985. However, when allowance is made for mean level shifts in the data for several of the stations aro und 1979, which may be due to calibration anti other instrument-relate d problems, the resulting overall trend is found to be of the order of +2% per decade and not statistically significant. An additional trend analysis using only RB meter data since 1979 at the 14 sites is also performed and leads to overall trend results similar to those from til e analysis which allows for mean level shifts in the data. The more de tailed analysis of data from four of the stations for the period 1979- 1991 is performed to investigate the extent to which the trend behavio r in the RB meter measurements call be explained by the behavior of ot her geophysical quantities such as cloudiness and total ozone. In part icular, radiative transfer model-based calculations of ultraviolet irr adiance based on satellite data from the total ozone mapping spectrome ter are compared with the RB meter measurements to help explain their behavior. Generally, inconsistencies are found between the trend behav ior in RB meter measurements and radiative transfer calculations, with the RB data showing substantial downward movement relative to the cal culations for three of the four sites. Significant evidence exists to indicate that problems with the network render the existing RB meter m easurements unreliable for long-term trend detection. Different reason able treatments of the data result in dramatically different trend res ults. Without further information, the data, by themselves, do not all ow for definitive trend analysis results.